A variety of cell lines have been evaluated for both constitutive and mitogen induced lymphokine production. Several human T cell hybridomas have also been tested. The human cell line RPMI-1788 has been confirmed to be a producer of macrophase migration inhibitory factor (MIF) and macrophage activation factor (MAF). However, we found some of the preparations contained interferon activity and determined that a portion of the biological activity of the RPMI-1788 produced lymphokines is due to the interferon. One preparation was found which contained no interferon but still had good MIF activity. The assays within the program for measuring MAF activity are being evaluated and standardized as a monitor for MAF production and purification by this project. Our results indicate that interferon may be involved in the mode of action of MIF and MAF and efforts to delineate possible sequential actions of these factors are being pursued.